Disk drives are capable of storing large amounts of digital data in a relatively small area. Disk drives store information on one or more recording media, which conventionally take the form of circular storage disks (e.g. media) having a plurality of concentric circular recording tracks. A typical disk drive has one or more disks for storing information. This information is written to and read from the disks using read/write heads mounted on actuator arms that are moved from track to track across the surfaces of the disks by an actuator mechanism.
Generally, the disks are mounted on a spindle that is turned by a spindle motor to pass the surfaces of the disks under the read/write heads. The spindle motor generally includes a shaft mounted on a base plate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the base plate to rotate the hub relative to the shaft. In order to facilitate rotation, one or more bearings are usually disposed between the hub and the shaft.
Over the years, storage density has tended to increase, and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage disks.
From the foregoing discussion, it can be seen that the bearing assembly that supports the storage disk is of critical importance. One bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disk hub. In addition to air, typical lubricants include gas, oil, or other fluids. The relatively rotating members may comprise bearing surfaces such as cones or spheres and comprise hydrodynamic grooves formed on the members themselves. Fluid dynamic bearings spread the bearing surface over a large surface area, as opposed to a ball bearing assembly, which comprises a series of point interfaces. This bearing surface distribution is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing, which helps to reduce non-repeatable run-out. Thus, fluid dynamic bearings are an advantageous bearing system.
Many current fluid dynamic bearing motor designs used in small form factor drives—that is, drives with stringent axial height constraints—suffer from insufficient angular stiffness due to the limited height/axial space available for journal bearing span. For example, conventional “single plate” fluid dynamic bearing motors are characterized by a journal bearing located on a shaft surface adjacent to a plate upon which two axial thrust bearings are formed. Since there are no features on the outer circumference of this thrust plate, its thickness (i.e., axial height) is essentially wasted space in the overall height budget of the motor. Similarly, vertically “stacking” the journal bearing and thrust plate results in a relatively tall bearing, which is difficult to fit in a small form factor motor while maintaining effective performance.
Further, it is well known in the art that angular stiffness is a function of the axial stiffness in a thrust bearing times a moment arm length (i.e., the thrust bearing diameter). However, as large diameter thrust bearings consume relatively more power, fluid dynamic bearing motor designs using large diameter thrust bearings to increase angular stiffness have been found to consume too much power to work effectively in small disc drives.
Thus, there is a need in the art for a fluid dynamic bearing design for small form factor disc drives that employs a large diameter thrust bearing that increases angular stiffness without consuming an impractical amount of power and a short journal bearing that conserves axial space.